The present invention relates generally to a radiation emitting device, and more particularly, to a method for delivering radiation treatment.
Radiation emitting devices are generally known and used, for instance, as radiation therapy devices for the treatment of patients. A radiation therapy device generally includes a gantry which can be swiveled around a horizontal axis of rotation in the course of a therapeutic treatment. A linear accelerator is located within the gantry for generating a high energy radiation beam for therapy. This high energy radiation beam may be an electron beam or photon (x-ray) beam, for example. During treatment, the radiation beam is trained on a zone of a patient lying in the isocenter of the gantry rotation.
In order to control the radiation emitted toward the patient, a beam shielding device, such as a plate arrangement or collimator, is typically provided in the trajectory of the radiation beam between the radiation source and the patient. An example of a plate arrangement is a set of four plates which can be used to define an opening for the radiation beam. The collimator is a beam shielding device which may include multiple leaves (e.g., relatively thin plates or rods) typically arranged as opposing leaf pairs. The plates are formed of a relatively dense and radiation impervious material and are generally independently positionable to delimit the radiation beam.
The beam shielding device defines a field on the zone of the patient for which a prescribed amount of radiation is to be delivered. The usual treatment field shape results in a three-dimensional treatment volume which includes segments of normal tissue, thereby limiting the dose that can be given to the tumor. The dose delivered to the tumor can be increased if the amount of normal tissue being irradiated is decreased and the dose delivered to the normal tissue is decreased. Avoidance of delivery of radiation to the healthy organs surrounding and overlying the tumor limits the dosage that can be delivered to the tumor.
The delivery of radiation by a radiation therapy device is typically prescribed by an oncologist. The prescription is a definition of a particular volume and level of radiation permitted to be delivered to that volume. Actual operation of the radiation equipment, however, is normally done by a therapist. The radiation emitting device is programmed to deliver the specific treatment prescribed by the oncologist. When programming the device for treatment, the therapist has to take into account the actual radiation output and has to adjust the dose delivery based on the plate arrangement opening to achieve the prescribed radiation treatment at the desired depth in the target.
The radiation therapist""s challenge is to determine the best number of fields and intensity levels to optimize dose volume histograms, which define a cumulative level of radiation that is to be delivered to a specified volume. Typical optimization engines optimize the dose volume histograms by considering the oncologist""s prescription, or three-dimensional specification of the dosage to be delivered. In such optimization engines, the three-dimensional volume is broken into cells, each cell defining a particular level of radiation to be administered. The outputs of the optimization engines are intensity maps, which are determined by varying the intensity at each cell in the map. The intensity maps specify a number of fields defining optimized intensity levels at each cell. The fields may be statically or dynamically modulated, such that a different accumulated dosage is received at different points in the field. Once radiation has been delivered according to the intensity map, the accumulated dosage at each cell, or dose volume histogram, should correspond to the prescription as closely as possible.
Methods for making the treatment volume correspond more closely with a tumor include defining the tumor shape with a lead alloy block, moving solid jaw blocks during treatment, scanning the radiation beam over the volume to be treated, and using a multi-leaf collimator to create an irregularly shaped field corresponding generally to the shape of the tumor. The multi-leaf collimator includes two opposing arrays of side-by-side elongated radiation blocking collimator leaves. Each leaf can be moved longitudinally towards or away from the central axis of the beam, thus defining a desired shape through which the radiation beam will pass. Multi-leaf collimators are increasingly being used to replace lead alloy blocks in many conformal radiation treatments to reduce costs and time required to create the block. However, there are still a number of treatment cases that require the use of blocks since conformal shaping can not be adequately accomplished using a multi-leaf collimator. This is due to a xe2x80x9cstair-stepxe2x80x9d effect that occurs along field edges that are not perpendicular to leaf face edges. An undulating dose pattern at the border of an irradiated volume results when the leaves are stepped to create an irregular shape. This distribution is unacceptable for field edges that are adjacent to critical structures or when abutment of additional fields is planned.
One method for reducing this stair-step effect is to divide the treatment dose into multiple intensity fields and shift the table supporting the patient between deliveries of each intensity field. However, this is often undesirable since the table shifts move the planned isocenter.
Another possible solution is to provide a collimator with thinner leaves. However, the hardware required for the additional leaves is expensive, adds weight to the system, may reduce clearance between the treatment head and the patient, and may decrease reliability and life of the system.
Accordingly, there is therefore, a need for a method for achieving higher spatial resolution intensity modulation to reduce stair-step effects at critical borders during radiation therapy without changing current multi-leaf collimator leaf widths or shifting the patient during radiation treatment.
A method for delivering radiation from a radiation source to a treatment area utilizing a multi-leaf collimator is disclosed. The method includes positioning the multi-leaf collimator between the radiation source and the treatment area to block a portion of the radiation. The leaves of the multi-leaf collimator extend longitudinally along a first axis and are positioned to define a first treatment field. The method further includes delivering radiation to the first treatment field and rotating the multi-leaf collimator about a central axis extending generally perpendicular to the leaf plane. The leaves are positioned to define a second treatment field and radiation is delivered to the second treatment field.
In one embodiment, the collimator is rotated until the leaves extend longitudinally along a second axis generally perpendicular to the first axis. The leaves may be moved longitudinally to create additional treatment fields. A prescribed radiation dose is preferably divided equally among the different treatment fields.
The method may further include dividing the treatment area into a plurality of cells, each having a defined treatment intensity level. The cells are grouped to form a plurality of matrices, each of the matrices having at least one dimension approximately equal to a width of the collimator leaf. Each of the matrices is decomposed into orthogonal matrices for delivery with a zero degree offset collimator and a ninety degree offset collimator.
The above is a brief description of some deficiencies in the prior art and advantages of the present invention. Other features, advantages, and embodiments of the invention will be apparent to those skilled in the art from the following description, drawings, and claims.